1. Field of the Invention
The present invention relates to a semiconductor device. More particularly, the present invention relates to a method for manufacturing a capacitor.
2. Description of the Related Art
Most capacitors that are formed in conjunction with other semiconductor devices have a pair of polysilicon electrodes. However, the polysilicon electrodes have some drawbacks including the formation of a depletion region whose thickness may vary. A variable thickness often leads to a variation of capacitance and a degradation of device performance. In the current state of technology, variation in depletion layer thickness often results in an even greater variation in capacitance because the inter-layer dielectric layer has become thinner due to miniaturization.
In general, the polysilicon electrodes of a capacitor are doped (for example, using arsenic or phosphorus ions) to increase electrical conductivity. However, when a voltage is applied to the capacitor, electric charges are induced at the junction between the electrode and the inter-layer dielectric layer. The electric charges near the junction cancel most of the effect of produced by the ionic dopants, thereby creating a depletion region.
The depletion region can be regarded as an extension of the inter-layer dielectric layer. The presence of the depletion region, therefore, increases the effective dielectric layer of the capacitor. In general, the charge storage capacity of a capacitor is inversely proportional to the thickness of the inter-layer dielectric layer. In other words, a capacitor having a thin dielectric layer is able to store a greater number of charges. However, the formation of a depletion layer increases the thickness of the dielectric layer, and hence reduces the capacitance of the capacitor. In addition, thickness of the depletion layer varies according to the voltage V applied to the electrodes. This can lead to a variation of the voltage coefficient (1/C(dC/dV)) of a capacitor and hence a de-stabilization of the device. Furthermore, polysilicon has a higher resistivity than other metallic materials. Therefore, polysilicon electrodes often limit the ultimate operating speed and performance of the capacitor.